Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W39: Focus Session: Negative Differential Resistance II |
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Sponsoring Units: FIAP Chair: Ravindra Pandey, Michigan Technological University Room: Colorado Convention Center 502 |
Thursday, March 8, 2007 2:30PM - 3:06PM |
W39.00001: Negative Differential Resistance in Insulating Systems: From Molecules to Polymers Invited Speaker: Swapan Pati We have developed a microscopic theory to explain the negative differential resistance behavior in molecular bridges. This feature has been observed in many molecules with different on/off ratios, sharpness of the current peak and the critical bias. Our theory, based on simple dimer model (both Peierls and donor/acceptor) together with bias driven conformational/ electronic change, covers almost all the experimental characteristics for a large number of real molecular systems and encompasses all the theory that has been known till date. Similar argument is also extended to Mott insulator, where we find a large number of insulator/quasi-metal transitions in finite size chains and a thermodynamic insulator/metal transition in polymers due to the application of static electric field between two ends of the chain. The interplay between charge inhomogenities and electric field induced polarization will be discussed in a number of cases. We will also show that none of these transitions follow Landau-Zener mechanism. I shall also discuss our theoretical proposal for the experimental strategies to stabilize highly unstable and reactive metal clusters like Al4Li4 and their analogs. \newline \newline Reference: \newline 1. S. Lakshmi and Swapan K. Pati, Phys. Rev. B 72, 193410 (2005). \newline 2. S. Lakshmi, Ayan Datta and Swapan K. Pati, Phys. Rev. B 72, 045131 (2005). \newline 3. S. Lakshmi and Swapan K. Pati, Spl on Nanosc and Tech, Pramana, 65, 593. (2005). \newline 4. S. Sengupta, S. Lakshmi and Swapan K Pati, J. Phys. Cond. Mat. 18, 9189 (2006). \newline 5. Swapan K. Pati and S. Ramasesha, J. Phys. Condens. Matter 16, 989 (2004). \newline 6. S.Lakshmi and Swapan K. Pati, J. Chem. Phys. 121, 11998 (2004). \newline 7. S. Dutta, S. Lakshmi and Swapan K Pati, Submitted (2006). \newline 8. A. Datta and Swapan K. Pati, J. Am. Chem. Soc. 127, 3496 (2005). \newline 9. Sairam S. M., A. Datta and Swapan K. Pati, J. Phys. Chem. B 110, 20098 (2006). \newline 10. A. Datta, Sairam S. M. and Swapan K. Pati, Acc. Chem. Res. (to appear) [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W39.00002: Bloch Oscillations of a Two-Dimensional Electron Gas in a Lateral Superlattice S.K. Lyo, W. Pan, J.L. Reno, J.A. Simmons, D. Li, S.R.J. Brueck We present theoretical result and experimental data for the DC current of a quasi-two-dimensional electron gas in a high electric field. The theoretical model considers inelastic scattering in a relaxation-time approximation and two-dimensional elastic scattering microscopically including inter-Bloch-band scattering in the degenerate and nondegenerate regime. The results show standard negative differential conductance. Inclusion of the effect of the electric field for the inelastic relaxation rate tends to flatten (i.e., saturate) the current after the peak current as a function of the field, yielding improved agreement between the theory and the observed data from modulated GaAs/Al$_x$Ga$_{1-x}$As quantum wells. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W39.00003: Nonequilibrium Kubo formula of a finite conductor connected to reservoirs obtained by the Keldysh formalism Tatsuya Fujii We show that the Keldysh formalism of the density matrix of a finite conductor attached to reservoirs has the MacLennan- Zubarev form. We point out that the Keldysh formalism describes the irreversible processes and the steady-state features of time-correlation functions. We find that the MacLennan-Zubarev form of the density matrix gives rise to a generalization of the Kubo formula into the nonequilibrium case. Based on it we propose a nonequilibrium identity between differential conductance, the noise power and the shot noise. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W39.00004: Length-Dependence of Electron Transfer in Conjugated Molecular Wires Shashi Karna, Govind Mallick, Ravindra Pandey The electron transfer (ET) properties of $\pi $-electron conjugated molecular wires consisting of polyene chain, [$>$C=C$<$]$_{n, (n=1-11)}$ has been investigated in the framework of \textit{ab} \textit{initio} molecular orbital theory. As expected, magnitude of the ET coupling matrix element V$_{DA}$ decreases exponentially with increase in the length of the molecular wire. However, in contrast with the rigid $\sigma $-bonded molecular wires, the decay constant, $\beta $, for the conjugated systems exhibits three different regimes over the calculated length. This is attributed to the delocalized nature of the electrons along molecular length that facilitates retention of the electron coupling even at large separations between the donor and acceptor centers. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 4:18PM |
W39.00005: Probing single molecule vibrations with the inelastic resonant tunneling Invited Speaker: Nikolai Sergueev Inelastic electron tunneling spectroscopy (IETS) has proven to be a valuable and powerful technique allowing identification and analysis of single molecular vibrations including those inaccessible by traditional optics measurements such as Raman and IR spectroscopies. Combined with scanning tunneling microscopy (STM) it provides a single molecule resolution. However, a comprehensive theoretical description of the electron coupling with molecular vibrations and the role it plays in conductance still remain a challenging problem. In this talk we present the first principles theory of the inelastic electron tunneling spectroscopy. Our method is based on density functional theory within Keldysh nonequilibrium Green's function formalism and allows us to treat electrons and molecular vibrations (phonons) on equal footing while computing electronic and vibrational spectrum, electron-phonon coupling, elastic and inelastic current in molecular electronic devices. The salient feature of our theory is that phonon effects on the electronic Hamiltonian are included in a self-consistent manner. Using this approach we investigate the effect of molecular vibrations on quantum transport through a C60 molecule contacted by two metallic electrodes. We demonstrate that its transport properties undergo significant changes when molecular vibrations are taken into account and show that this effect mostly originates from the resonance nature of the quantum tunneling which is expected to be true for the vast majority of the metallic electrodes. We also report a vibrational spectroscopy analysis and report those vibrational modes that contribute most to the inelastic quantum tunneling. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W39.00006: Universal Negative Differential Resistance in Single-Electron Transport through Atoms and Molecules Nikita Simonian, Jingbin Li, Konstantin Likharev We have carried out numerical calculations of single-electron transport through single atoms and organic molecules (OPE chains terminated with isocyanide groups), weakly coupled to gold electrodes. The calculations were based on the general theory of single-electron tunneling in systems with discrete energy spectrum [1], with molecular orbitals obtained by the ab initio DFT solver NRLMOL [2]. The Kohn-Sham potential calculated by the solver was also used to calculate the wavefunctions of ``external'' electrons, so that the necessary overlap integrals could be obtained using the Bardeen formula [3] rather from the NEGF approach. The most remarkable result of the calculations is the virtually unversal negative differential resistance, due to a new physical mechanism resulting from the suppression of transparency of one of the tunnel barriers of the system by the applied source-drain electric field. The work is supported in part by AFOSR and NSF. [1] D. V. Averin, A. N. Korotkov, and K. K. Likharev, Phys. Rev. B 44, 6199 (1991). [2] See http://cst-www.nrl.navy.mil/$\sim$nrlmol/. [3] J. Bardeen, Phys. Rev. Lett. 6, 57 (1961). [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W39.00007: Ab initio RTM/NEGF method for Electron Transport through Single Molecules Kenji Hirose, Nobuhiko Kobayashi Using the RTM/NEGF method, which is an ab initio calculation method based on the density-functional formalism with use of accurate scattering waves combined with non-equilibrium Green's function method, we study the transport properties between metallic electrodes through single molecules. Especially, we investigate how the atomic-scale contacts to electrodes affect quantum transport. We find that transport behaviors change significantly due to the contacts. For fairly good contacts, transport properties are determined by the HOMO-LUMO states by resonant tunneling processes. However, as the contacts to one electrode becomes worse, I-V characteristics are mostly determined by tunneling condition with strong non-linear behaviors and molecular states are hard to be observed in the conductance data. Furthermore, we find that negative differential resistance appears at some distances between single molecules and one of the electrodes. We will clarify the mechanisms for these anomalous transport behaviors and show the relationship of HOMO-LUMO resonant states and tunneling vs. ballistic transport with various contact conditions to electrodes. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W39.00008: Negative differential resistance in molecular conductors Mortko Kozhushner, Ivan Oleynik Negative differential resistance $\sigma _{d}$=dI/dV$<$0 has been observed in numerous experiments that measured conductance through organic molecules. We will discuss the fundamental mechanism of negative differential resistance in molecular conductors. The origin of NDR is in the nontrivial evolution of resonant conductance states of a molecular conductor in external electric field. These resonant states are the states of positive and negative molecular ions (electron and hole states). In an electric field, the resonant levels are lowered and their wave functions become assymetrical. This results in an initial increase of the resonant molecular current as a resonant level appears in the interval of energies [E$_{F}$, E$_{F}$-eV] and a subsequent decrease of the current as the asymmetry in localization at the right and left interfaces is amplified upon further increase of applied bias. [Preview Abstract] |
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